Hybrid vehicles, electric vehicles and other similar motored vehicles capable of generating electric power while they are traveling have a secondary battery, an electric double layer capacitor or a similar power storage device mounted therein. For example, the hybrid vehicle has a motor generator converting the electric power that is stored in the power storage device into a driving power which is in turn transmitted to a wheel singly or together with a driving power that the engine generates.
A secondary battery stores electrical energy by utilizing a chemical reaction. Accordingly, its charging and discharging characteristics significantly vary with environmental factors, temperature conditions in particular. More specifically, at low temperatures, the chemical reaction has a significantly decreased level of reactivity and the secondary battery may not be able to discharge sufficient electric power. As such, in early winter morning or in cold climate areas, the secondary battery can only supply decreased electric power, and in a hybrid vehicle configured to have a motor generator crank the engine to start, for example its vehicular system including the engine cannot be started. Such temperature dependence is exhibited more significantly in particular by a lithium ion battery than a nickel metal hydride battery, as the former has a higher energy density than the latter.
Furthermore the electric double layer capacitor is also temperature-dependent and can only supply decreased electric power at low temperatures.
Accordingly, there has been proposed a controlling method for charging and discharging. More specifically, after an ignition-off command is received, a temperature condition that would be assumed when a subsequent ignition-on command is received is estimated, and a power storage device is charged to a higher SOC to ensure that a vehicular system can also start under the estimated temperature condition.
For example, Japanese Patent Laying-Open No. 11-355967 discloses a battery control apparatus improving an engine's startability at low temperatures. This battery control apparatus includes: temperature estimation means for estimating the temperature that a battery has when an engine next starts; SOC setting means setting the battery's SOC corresponding to the battery's estimated value in temperature for obtaining a predetermined output from the battery; SOC detection means for detecting the battery's SOC; and means for controlling charging and discharging the battery to allow the battery's detected SOC value to attain the set SOC value. The temperature estimation means estimates the temperature that the battery has when the engine next starts, as based on the value of an outside air temperature detected when the engine was previously started. The battery control apparatus can provide an output required to start the engine if the battery has a low temperature when the engine is next started.
However, while Japanese Patent Laying-Open No. 11-355967 discloses that a temperature that the battery has when the engine next starts is estimated from the value of an outside air temperature detected when the engine was previously started, the publication does not specifically disclose how the temperature that the battery has when the engine next starts is estimated from the value of the outside air temperature detected when the engine was previously started.
Thus if there is some correlation between the value of the outside air temperature detected when the engine was previously started and the temperature that the battery has when the engine next starts, it has been difficult for a skilled person to implement the battery control apparatus of Japanese Patent Laying-Open No. 11-355967.